Polar or non-polar? Syntheses, crystal structures, and optical properties of three new palladium(II) iodates

Three new novel palladium(II) iodates with square-planar PdO(4) units, namely, Pd(IO(3))(2), AgPd(IO(3))(3), and BaPd(IO(3))(4), have been successfully hydrothermally synthesized. They represent the first ternary and quaternary palladium(II) iodates and display three different structural types. Pd(IO(3))(2) exhibits a novel two-dimensional (2D) layered structure in which each PdO(4) square further connects with four neighboring ones by four bridging IO(3) groups. AgPd(IO(3))(3) exhibits a unique three-dimensional (3D) network based on unique one-dimensional (1D) [Pd(IO(3))(3)](-) anionic chains along the c-axis which are further interconnected by Ag(+) cations. BaPd(IO(3))(4) is isostructural with KAu(IO(3))(4), and its structure features zero-dimensional (0D) [Pd(IO(3))(4)](2-) anionic units that are interconnected by Ba(2+) cations. These materials can be polar or non-polar depending on the different alignments of the lone pairs of the I(V) atoms. Pd(IO(3))(2) and AgPd(IO(3))(3) are non-polar and centrosymmetric, hence not second-harmonic generation (SHG) active. BaPd(IO(3))(4) is polar and displays a moderate SHG response of about 0.4× KTP. Thermal analyses, optical, luminescent, and ferroelectric properties as well as electronic structure calculations have also been performed.